Alkali Metal Halide Effect

Closely related to these investigations, Breslow and co-workers studied the Diels-Alder reaction of CP with methyl vinyl ketone (MVK) in water-like solvents, ethylene glycol and formamide, in the presence of lithium salts. They found clear differences and similarities between water and these two solvent systems. In the absence of Li salts, the second-order rate constant for the reaction at 20 °C increased in formamide ( 2 = 3184 X 10 m s ), and even more in ethylene glycol (480 x 10 m s ), relative to a polar solvent such as methanol (75.5 x 10 m s ) or non-polar solvent such as isooctane (5.940.3 x 10 m s ). The reactions in both polar solvents were faster in the presence of LiC104 than in the presence of LiCl, although the perchlorate ion has less salting-out effect than chloride ion in water [41]. 

In the enolization of 3-pentanone by lithium 2,2,6,6-tetramethylpiperidide (LTMP), kinetic ElZ selectivity normally obtained in THF at low temperature is only about 5 1, whereas in the presence of 0.3-0.4 equiv. LiCl, this ratio increases to 50-60 1. Surprisingly, with large quantities of LiCl (1 equiv.), the selectivity returned to ca 10 1 (Sch. 12) [46], 

Collum and co-workers investigated the aggregation structure of LDA with LiX. At high LiCl concentrations the mixed-dimer structure MD-1 is the major LDA species at lower concentrations two distinctive structures are possible, the mixed-cyclic trimer MCT-1 and the ladder-trimer LT-1 (Fig. 4) [47]. Rigorous establishment of the presence of LTMP-LiX aggregates has not been forthcoming, because the various LTMP 

There are several examples of the effect of LiX on enolate aggregation leading to increased enantiomeric excess in asymmetric chemical events. Koga and co-workers developed an efficient enantioselective benzylation of the lithium enolate of 19 by using a stoichiometric amount of chiral ligand 22 with LiBr in toluene [50]. The chiral lithium amide 22 was prepared by treatment of a mixture of the corresponding amine 21 and LiBr in toluene with a solution of n-BuLi in hexane. Sequential addition of ketone 19 and benzyl bromide gave rise to 20 in 89 % yield and 92 % ee. The amount 

Enantioselective benzylation of ketone 19 gives further insight into the LiX effect (Sch. 13). In the absence of LiBr, the amount of ee is time-dependent, increasing as reaction time is increased. This phenomenon can he rationalized in terms of the effect of LiX which is gradually formed as the reaction proceeds, and which is assumed to involve conversion of a poorly selective aggregate into a much more selective mixed aggregate. 

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The pellet (pressed-disk) technique depends on the fact that dry, powdered potassium bromide (or other alkali metal halides) can be compacted under pressure in vacuo to form transparent disks. The sample (0.5-1.0 mg) is intimately mixed with approximately 100 mg of dry, powdered KBr. Mixing can be effected by thorough grinding in a smooth agate mortar or, more efficiently, with a small vibrating ball mill, or by lyophili-... 

The first general comment relates to the solvent system. In those cases where the electrolysis substrate does not exist in an aqueous-ethanolic or methanolic solution in a suitable ionic form, it is necessary to provide a solvent system of low electrical resistance which will dissolve the substrate, and also a supporting electrolyte whose function is to carry the current between the electrodes. Examples of such solvents are dioxane, glyme, acetonitrile, dimethylformamide and dimethyl sulphoxide supporting electrolytes include the alkali metal halides and perchlorates, and the alkylammonium salts (e.g. perchlorates, tetrafluoro-borates, toluene-p-sulphonates). With these electrolysis substrates, mass transfer to the electrode surface is effected by efficient stirring. 

Other cases of approximately monatomic chromophores occur in 4f-+5d transitions now known in Sm11, Eu11, Tm11,28 Ybn, Cera, Prm, and Tbra.16 (The half-filled shell effect expressed by Eq. (3) is very conspicuous in this distribution of known species.) 5transitions are known in UIU, Np111, Puin, Paiy, U, Np, and Pu17. 5s-+5p transitions are known in complexes of Snn and Sbm and 6s — 6p in Tl1, Pbn, and Bira. The halide ions in solvents of not too high electron affinity and in crystals of alkali metal halides show absorption bands which to a certain approximation can be described as 3p - 4s(Cl), 4p — -5s(Br), and 5p - -6s(I). 

The pellet (pressed-disk) technique depends on the fact that dry, powdered potassium bromide (or other alkali metal halides) can be compacted under pressure to form transparent disks. The sample (0.5-1.0 mg) is intimately mixed with approximately 100 mg of dry, powdered KBr. Mixing can be effected by thorough grinding in a smooth agate mortar or, more efficiently, with a small vibrating ball mill, or by lyophilization. The mixture is pressed with special dies under a pressure of 10,000-15,000 psi into a transparent disk. The quality of the spectrum depends on the intimacy of mixing and the reduction of the suspended particles to 2 gm or less. Microdisks, 0.5-1.5 mm in diameter, can be used with a beam condenser. The microdisk technique permits examination of samples as small as 1 fxg. Bands near 3448 and 1639 cm-1, resulting from moisture, frequently appear in spectra obtained by the pressed-disk technique. 

Rare earth oxides are useful for partial oxidation of natural gas to ethane and ethylene. Samarium oxide doped with alkali metal halides is the most effective catalyst for producing predominantly ethylene. In syngas chemistry, addition of rare earths has proven to be useful to catalyst activity and selectivity. Formerly thorium oxide was used in the Fisher-Tropsch process. Recently ruthenium supported on rare earth oxides was found selective for lower olefin production. Also praseodymium-iron/alumina catalysts produce hydrocarbons in the middle distillate range. Further unusual catalytic properties have been found for lanthanide intermetallics like CeCo2, CeNi2, ThNis- Rare earth compounds (Ce, La) are effective promoters in alcohol synthesis, steam reforming of hydrocarbons, alcohol carbonylation and selective oxidation of olefins. 

Syntheses that exploit the solubility of the alkaline-earth metals in liquid ammonia have proven practical for alkoxide work, as they generate high yields, reaction rates, and purity (Table 8, Equation (3)). In a refinement of this approach, Caulton and co-workers have used dissolved ammonia in an ethereal solvent, usually THF, to effect the production of a number of alkoxides of barium, and this method has also been examined with calcium and strontium (Table 8, Equations (4a) to (4c)). Displacement reactions using alkali metal alkoxides and alkaline-earth dihalides (Table 8, Equation (5)), and between alkaline-earth hydrides or amides and alcohols (Table 8, Equations (6) and (7)), have been examined, but alkali-metal halide impurities, incomplete reactions, and unexpected equilibria and byproducts can affect the usefulness of these approaches. 

According to the Lorentz-Lorenz equation (4.3.21) for the molar refraction at optical frequencies, Y is directly proportional to the molecular polarizability p. The Koppel-Palm equation has also been applied to the analysis of solvent effects on thermodynamic quantities related to the solvation of electrolytes [48, 49]. In the case of the systems considered in table 4.11, addition of the parameter X to the linear equation describing the solvent effect improves the quality of the fit to the experimental data, especially in the case of alkali metal halide electrolytes involving larger ions. The parameter Y is not important for these systems but does assist in the interpretation of other thermodynamic quantities which are solvent dependent [48, 49]. Addition of these parameters to the analysis is only possible when the solvent-dependent phenomenon has been studied in a large number of solvents. 

For the alkali metal halides the influence of the anion is practically non-existent as can be seen from the comparison of RbCl, RbBr and Rbl (Table 3). In the more covalent structures both atoms appear to be effected by increase in pressure, as changes in the band structure lead to an increase in conductivity. 

The composition of the electrolyte is of primary importance owing to the required high ionic conductivity, compatibility with active and inert cell components, wetting characteristics, effects on the course of electrode reactions, and the cell s cost. The optimized electrolyte mixtures consist of alkali-metal halides. An all Li ion electrolyte would be preferred from the point of view of kinetic and transport considerations however, the decreased specific energy and high cost are then prohibitive. 

One of the most interesting applications of the HSAB concept consists in the prediction of the stability of the complexes formed owing to interaction of alkali metal halides with rare-earth metal halides. These systems are of great interest for the materials science of scintillation materials the said complex halides are now considered among the most promising scintillation detectors and sensors. Besides, the Li- and Gd-based materials are especially convenient as effective detectors of thermal neutrons. The compositions and stability of the formed compounds depend considerably on the kind of acids and bases from which the compound is formed. So, Li+ cation is one of the hardest cation acids, and, therefore, the formation of stable complex halides of Li and lanthanides according to reaction ... 

Silicon dioxide, one of the products of this interaction, is insoluble in pure alkali metal halides and separates from the molten medium owing to the difference in densities. Thermodynamic analysis of the processes of molten iodide purification with different halogenating agents shows that their effectiveness reduces in the sequence SH4 > HI >h [294], An obvious advantage of silicon halides for the purification of halide melts used for singlecrystal growth is the fact that their use does not result in the appearance of additional impurities in the purified melts, since these processes are usually performed in quartz (Si02) vessels-reactors. 

The substitution of the melt-anion in a molten alkali-metal halide, i.e. proceeding from chloride melts to bromides and iodides, results in the appreciable reduction in the contribution of the ionized constituent (Me2+ concentration) in the total oxide solubilities in the said melts. The data allow us to trace the effect of the constituent halide ion on the solubilities of metal-oxides in the corresponding melt. It should be noted that the substitution of chloride ions with bromide ones results in an appreciable reduction in the solubilities of all the metal-oxides studied, i.e. the values of pPMeo N become lower by 1-2 orders. At the same time, the oxide dissociation in the saturated solution of the oxide reduces. Thus, a considerable reduction in metal-oxide solubilities in bromide melts as compared with chloride ones can be explained in a similar way. Equilibrium (2.4.13) which takes place at the dissolution of a metal-oxide in an ionic halide melt is superimposed upon the interactions of the metal cation with the anions of the melt-solvent, which are denoted hereafter as X (equation (3.6.3)). 

From the above-mentioned studies [374, 375] it can be concluded that, up to now, systematic solubility investigations in molten potassium halides have not been performed. This also concerns other alkali-metal halide melts, although these results would be very useful for the estimation of the effect of melt acidity and anion composition on metal-oxide solubilities at 800 °C. 

In order to estimate the effect of the cation composition of melts based on alkali-metal halides upon the metal-oxide solubilities, we investigated the... 

Solution state standards for each of the quadrupolar halogen nuclei were also recommended (Table 1). As mentioned previously, one should carefully note both the concentration and solvent when preparing solution state standards, as the measured shift values have been shown to depend significantly upon these two variables (see also Section 4.5). Typical solvent isotope effects (expressed as the result of 5 X, H2O)—5(X, D2O), X = Cl, Br, I) are ca. 5 ppm for chlorine, 8-10 ppm for bromine, and 13 ppm for Na I, and are one (in the case of Cs) to three (Li/Na) orders of magnitude greater than the corresponding solvent isotope shifts for the alkali metals in the alkali metal halides. 

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